1. Field of the Invention
The present invention relates to communication systems, in particular, to asynchronous transfer mode processing of packets.
2. Description of the Related Art
Asynchronous transfer mode (ATM) is a switching technique for telecommunications networks, and is commonly used for virtual private networks (VPNs). ATM establishes a virtual circuit (VC) between two endpoints before beginning data exchange and uses asynchronous time division multiplexing. ATM encodes data into short, fixed length packets referred to as cells. An ATM cell comprises a 5 byte header which is used by a network to deliver a cell to its destination, and a 48 byte payload which may represent part of voice, video, or data transmission. The cell header also comprises a Virtual Path Identifier (VPI) and a Virtual Channel Identifier (VCI). The VPI and the VCI identify the VC used by a connection. As cells traverse an ATM network, switching takes place by changing the VPI/VCI values.
The upper layer of the ATM is the ATM Adaptation Layer (AAL). The AAL supports upper layer information transfer protocols that are not based on ATM. The AAL defines how to segment and reassemble higher-layer packets into ATM cells, and how to handle various transmission aspects in the ATM layer. Services requiring adaptation include Gigabit Ethernet, Internet Protocol (IP), and UMTS/Wireless. The International Telecommunications Union (ITU) has defined several AAL protocols to meet a variety of needs. The AALs are classified into types based on whether a timing relationship must be maintained between source and destination, whether the application requires a constant bit rate, and whether the transfer is connection-oriented or connectionless. For example, AAL Type 2 (AAL2) supports time-dependent Variable Bit Rate (VBR-RT) of connection-oriented, synchronous traffic, such as voice over ATM. AAL Type 5 (AAL5) is commonly used in data traffic, such as IP. Details on AAL2 segmentation and reassembly are described in “ITU Telecommunication Standardization Sector (ITU-T) Recommendation 1.366.1; Segmentation and Reassembly Service Specific Convergence Sublayer for the AAL Type 2”, hereinafter referred to as “ITU-T I.366.1”.
In general, segmentation and reassembly refers to fragmenting and reassembling variable length packets into fixed length cells in order to transport them across ATM networks. The segmentation process differs according to the AAL type, but generally AAL traffic superimposes multiple minipacket “conversations” in ATM cells (e.g., 53 byte ATM packets) so that ATM cells include interleaved AAL minipackets. Nearly every packet from other protocols must undergo segmentation so that it complies with an ATM payload of 48 bytes. Packets are generally reassembled at their destination so that the original packet is reconstituted. Processing interleaved AAL traffic consumes many resources of the destination processor, and some types of packet reassembly are particularly demanding, limiting overall throughput of a network. For example, each AAL conversation often requires its own reassembly queue within the destination processor to reassemble and reorder the AAL minipackets.